Manufacturers are using stereolithography to improve product quality, to reduce expenditures and minimize the time required to bring new products to market.
One step in the introduction of a new product is the making of a model or prototype. A manufacturer needs a physical model of new or improved parts or products before it commits to production. The model allows people to physically handle the object, measure it, and detect design flaws early in the design process. Every year thousands of prototypes are made for parts that go into, for example, an automobile, airplane, or missile, the packaging of computers and other electronic systems, the components of consumer appliances, and dispensers for such products as perfumes, detergents, and shampoos--almost everything that is produced for personal use or for business. Traditional methods of making prototypes involve expensive and time-consuming manual procedures, or complex programming for numerically-controlled machine tools. Stereolithography has reduced the cost of prototypes and the time needed to make them. Many service bureaus make prototypes for companies that do not have their own stereolithographic equipment.
Stereolithography can produce short runs of certain finished products in the final material configuration directly from a computer aided design ("CAD") software. Such parts have been used as a stopgap measure to introduce products until tooling has been perfected for conventional volume production parts. A current limitation on the direct manufacture of production parts is the lack of availability of proper photopolymers. Intense research and development work is going on to develop materials that could closely resemble the final material properties. Once the appropriate photopolymers are developed, manufacturing of actual production parts could be the main use for photopolymers.
Stereolithography is a way to make solid objects by successively "printing" thin layers of a cured photopolymer, one on top of the other. A number of ways are known for accomplishing this end.
For example, U.S. Pat. No. 4,575,330 issued Mar. 11, 1986 to Hull describes a scanning method for stereolithography. A concentrated beam of ultraviolet light is focused on the surface of a container filled with a liquid photopolymer. The light beam, moving under computer control, draws a layer of the object onto the surface of the liquid. Wherever the beam strikes the surface, a very thin layer of the photopolymer reacts by polymerizing or crosslinking, and changes into a solid. To make a three-dimensional object, the entire operation is performed again and again, with the position of the object shifted slightly each time, so that the object is built up layer by layer. Very precise control of the light source is required, so a computer-controlled laser is used. The same is true of the position of the object, which is typically shifted downward in the container in small increments.
A computer-aided design, manufacture and engineering ("CAD/CAM/CAE") software mathematically slices a three-dimensional computer model of the object into many thin layers. The software controls the motion of a laser beam across the surface of the polymer and also the steplike position shifts of the formed object. The laser is focused in a tiny area and repeatedly scans across the surface of the liquid, leaving a pattern of cured and uncured areas in much the same way that light and dark points are made to produce a picture on a television screen. This type scanning is called raster scanning.
When a part has a freestanding section or overhang, a support structure is designed using the CAD program. When the part is completely built, it is removed from the container, heated to drain off excess liquid, and, if necessary, further cured in an oven. The supporting structure is cut away, and the part may be painted or surface-finished.
Another method of stereolithography involves the use of a photomask to build objects. In that method, a high power UV lamp is used to flood expose one layer of a liquid photopolymer at a time through a negative, or mask. The mask is generated electrostatically on a glass plate with a toner powder. A 2-second exposure from the lamp solidifies a thin surface layer of the photopolymer. The exposed mask is physically wiped clean and electrostatically discharged to prepare it for the next cross-section image. At the same time, the uncured photopolymer, which is still liquid, is blown (air-knifed), vacuumed or washed away. The cavities left by the uncured polymer are filled with hot wax. The wax solidifies to form a support structure for the next layer. Finally, the entire surface is milled with a cutter to make it ready for the next polymer layer. The cycle is repeated, so that the object is built up layer by layer.
Stereolithography has also been used to produce wax patterns indirectly or produce resin patterns directly. A conventional method involves creating a master pattern, then producing a die and injecting the die with wax. The wax model is then coated with a porous ceramic slurry, creating a mold. The wax is melted out and metal is poured in the `investment` mold. The mold is later broken or washed away, leaving the desired metal part. In this process, the resin pattern produced by stereolithography can replace wax and is burned rather than melted. A typical process involves heating the shelled pattern from ambient temperature to 100.degree. C. over a 24-hour period so that there is an overall volume reduction, and subsequent burnout at 750.degree.-900.degree. C. This investment casting method is described in U.S. Pat. No. 4,844,144 issued to Murphy et al., Jul. 4, 1989.
Stereolithography is also available as an alternative to room-temperature-vulcanized molds, which are made out of silicone rubber and are widely used to make prototypes as well as production parts in such industries as aerospace, sporting goods, toys and decorative plastic furniture. It customarily takes about eight weeks to make a conventional pattern and the mold. With stereolithographic patterns, this time can be reduced to three to five days. The photopolymer masters produced by stereolithography are also used to make hard tooling for injection molding and blow molding. Typically, the masters are coated with arc-sprayed atomized metal and subsequently built up with reinforced epoxies or similar hard compositions. Such molds are used for prototyping or limited production runs.
In other applications, stereolithography offers new advantages that were simply unavailable before. In the medical field, CAT scan data can be used to create 3-D models of damaged body parts. By manipulation and measurement of the model, a surgeon can devise more effective treatment of an injury, reducing the need for multiple surgeries in complicated cases such as hip replacements, and decreasing recovery time.
Other methods of building objects using separately cured layers of photopolymers are also available, and new methods and equipment are expected.
One of the basic limiting requirements for any stereolithographic method is the photosensitive resin. The present invention focuses on an improved resin, which exhibits enhanced photospeed. A number of photosensitive resins are now available for use with stereolithographic equipment. Current stereolithographic applications require resins that are mechanically stable, with a resolution of 0.005 inch. The photopolymer must cure rapidly and also should be able to endure both cure and post-cure treatments with minimal distortion. At present it appears that there is some trade-off between speed of cure, or photospeed, and size stability during cure. The present invention exhibits greatly enhanced photospeed as well as size stability comparable with the fastest commercially available resins.
As photopolymer technology develops, various test criteria have begun to evolve. One such criterion is related to photospeed and is called E.sub.c, the critical exposure energy, which is conveniently defined in "Rapid Phototyping & Manufacturing; Fundamentals of Stereolithography" P. F. Jacobs, Society of Manufacturing Engineers (1992), pp. 33, lines 14-16. This E.sub.c is the minimum exposure needed to induce polymerization and is measured in millijoules per square centimeter (mJ/cm.sup.2). A standard technique for determining E.sub.c is the WINDOWPANE.RTM. technique, also described in "Rapid Prototyping & Manufacturing", above, at pages 24-29. For the inventors' purposes, an E.sub.c of less than 1.5 is desirable.
Various resins have been explored for use in stereolithography. For example, International Publication No. WO 92/20014 relates to polymer compositions for stereolithography that contain vinyl ether-epoxide polymers. These compositions show good accuracy, but are extremely slow. That is, they have an E.sub.c of 27. They are also sensitive to ambient humidity.
U.S. Pat. No. 5,167,882 issued to Jacobine et al. Dec. 1, 1992, relates to norbornene/thiol and free radical initiators. These materials are disclosed to lack stability (Col. 3, lines 46-51).
International Publication No. WO 92/02572 relates to an associative blend of electron-donating and electron-accepting groups, principally a mixture of polymeric and monomeric (meth)acrylates with free radical initiators and an inert thermoplastic material. These materials are disclosed to be especially useful for investment casting.
U.S. Pat. No. 4,942,001, issued to Murphy et al. Jul. 17, 1990 relates to polymeric (meth)acrylates blended with an N-vinyl monomer. The incorporation of an N-vinyl monomer such as N-vinyl pyrrolidone is undesirable, as this compound is a suspected carcinogen. It is disclosed that exposure to preferably 0.2 to about 5 Joules per square centimeter (Col. 2, lines 65-66) will partially cure the blend, resulting in gelatinous and mechanically weak object (Col. 3, lines 3-9) and is subject to further exposure to radiation or thermal cure. (Col. 4, lines 32-47).
Various publications relate to (meth)acrylate formulations and free-radical photopackages. EP 506616 corresponding to Canadian Patent No. 2,063,982 relates to polyurethane (meth)acrylate and various (meth)acrylates and contains no photosensitivity data; EP 517657 relates to polyester (meth)acrylates; EP 378144 relates to (meth)acrylated epoxies and discloses that unspecified additives, including oxygen scavengers, can be used; and EP 450254 relates to mixed free-radical and ionic photoinitiators with polymeric and monomeric (meth)acrylates and uses lasers with wavelengths greater than 400 nm. The commercially available materials from these classes can be obtained from Ciba, all of which have an E.sub.c &gt;4 in the range of 300-400 nm.